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        // Symbol  bigInt
        // ES6  字符串  let  const var  模板字符串 `${a}` str.repeat()
        // 数组  [...arr]  function(...arg){}  let [a,b] = [1,2];
        // 对象 let obj = {a:a,fn:function(){}}--->
        // let obj = {a,fn(){}}  Object.keys  Object.values Object.assign
        // {...obj}   let {a,b} = {a:1,b:2}
        // promise --->手写一个promise
        // MutationObserver
        // Promise.then  Promise.resolve  Promise.reject all allSettled  catch..
        // async await
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